Which Diff Ratio to Use??

[porschelou]

We are headed to Watkins Glen in April for some racing. The car is a 86 325e with a 3.5 b34/m30 motor(roughly 200hp and good TQ #'s). We have been using a 3.73 LSD gear ratio for the last many years. At VIR we noticed on the straights we don't rev out and can only hit about 121mph - need a lower gear to get up to speed faster. So, we ran out and got a used 4.10 LSD but now we think that might be too low. If we do the math:
Our Transmission gear ratio’s

1-3.83
2-2.20
3-1.40
4-1.00
5- .81


Current 3.73 ratio

RPM MPH
5000 113
5500 124
6000 136


3.91 ratio

RPM MPH
5000 108
5500 119
6000 130


4.10 ratio

RPM MPH
5000 103
5500 113
6000 124

We have never run WGI before but watching video's it's a very fast track. It seem like a 4.10 might be too much. 3.91 seems like a better choice. Looking for some expert opinions out there. We have started the search for a good used 3.91 in case that's the direction we need to go.
Thanks

[seatown88]

what size wheels/tires are you running? It seems like you may want the 3.91 diff, or switch tire size.

[ukrbmw]

WG is similar in top speed to VIR.

Do you have data - TM or anything similar? Sometimes speedo can be off in terms of what speed you are actually running. 3.91 does seem to be a middle ground, however 4.10 is probably easier and cheaper to get (unless you already have a 3.91 sitting at home). Other option as the person above mentioned is to go to a slightly taller tire and 4.10 so it would be almost like a 3.91.

[porschelou]

We use 15' wheels with 205/50/15's. We don't have any real data. We have a 4.10 but have started shopping for 3.91. We will have a test/tune day prior to the races. This would give us an opportunity to swap diff's if needed. I wish we could change aspect ratio on the tires but we are limited to which tire we can run.

Thanks for the info..

[TobyB]

I'd do the 3.91, just from your calculations...

t

[Emre]

WGI is one of my "home tracks" (along with Mosport and Tremblant). If you'd like any tips, feel free to ask here or by PM.

With a stock M20B25 engine and 225/45-R15 tires, the 4.10:1 LSD is perfect. The shift points line up exactly where you want them to be in order to put the power down coming out of the critical corners.

With an M30B35 and the same-sized tires, I would expect the 3.73:1 LSD to be just about right. Personally, I wouldn't bother with the swap.

At VIR we noticed on the straights we don't rev out and can only hit about 121mph - need a lower gear to get up to speed faster.

If you try to gear the car such that you'll max out in 5th gear at the top of the back straight, you'll be in-between gears in some of the critical corners. I really don't think it's worth it.

We use 15' wheels with 205/50/15's.

That's a little puzzling. You can easily fit 225/45-R15 under the stock wheelarches. That extra width really makes a difference in the high-speed corners.

We have a 4.10 but have started shopping for 3.91

The difference between the 4.10:1 and 3.91:1 is so small that you can easily make up the difference by playing with tire size. Assuming you're starting with the 225/45-R15 (which you should be!), all you need to do if you want slightly taller effective gearing is to switch to 225/50-R15 rubber.

No need to hunt for a 3.91:1 rear end. They're hard to find and good ones are expensive.

[djjerme]

Go with the shorter tire and 4:10, the shorter sidewall should give you a little more response/feel in the corners/braking zones.

[yeaseth]

my orignal ratio was 3.73, then I ran a 3.91 for a while and didnt like how short it was so I switched back to the 3.73

[Jparkr]

Go with the shorter tire and 4:10, the shorter sidewall should give you a little more response/feel in the corners/braking zones.

Our pro3 15s with 50mm sidewalls keep the 4:10 ratio, when we run on 45mm sidewalls it turns into a 4:27 if I remember the rule meeting convo right.

That being said OP I would run a 3.74 with a smaller sidewall to make a 3:9x ratio.

We also have an M30b35 chumpcar and run a 4:10. More than happy with it Straight away speed is ok but power exiting corners will make you much more competitive.

[Fusion]

Which 4.10 will take the M50 or M30 power? I hear the small case LSDs will pop.

[Emre]

Which 4.10 will take the M50 or M30 power? I hear the small case LSDs will pop.

Small case (from the '91 318is) won't survive an M20, let alone an M30 or M50. Ask me how I know!

If you want to swap, you'll need a medium-case 4.10:1 LSD from an E30 M3.

[Emre]

That being said OP I would run a 3.74 with a smaller sidewall to make a 3:9x ratio.

Agreed. The OP should keep his 3.73:1 LSD and run 225/45-R15 tires on 15x8" wheels.

[ForcedFirebird]

Small case (from the '91 318is) won't survive an M20, let alone an M30 or M50. Ask me how I know!

If you want to swap, you'll need a medium-case 4.10:1 LSD from an E30 M3.

Any 318is that was originally purchased with the optional LSD came in medium case, same with the automatics. My auto was ordered with LSD (auto came with 4.10), swapped in a manual and am using it on the track. Homestead is a shorter track than the above mentioned ones, so difficult to give advice.

[M-technik-3]

Small case (from the '91 318is) won't survive an M20, let alone an M30 or M50. Ask me how I know!

If you want to swap, you'll need a medium-case 4.10:1 LSD from an E30 M3.

Or a convertible automatic.

[Wh33lhop]

Any 318is that was originally purchased with the optional LSD came in medium case.

I think you are wrong on this. Medium case 4.10s can be found on (some) auto 325is and M3s. 318is all came with small case 4.10s, some open, some fun edition.